Illumination structure including cavity and tir structure

ABSTRACT

A thin plastic light guide is formed to have cavities on one surface and TIR (total internal reflection) structures directly above the cavities. LEDs mounted on a printed circuit board are positioned hi the cavities. The cavity walls are shaped to refract the LED light and direct the LED light toward the TIR structures to most efficiently make use of the TIR structures. The TIR structures may have a cusp or cone shape. The top ceiling of the cavities may be shaped to direct light at the TIR structure so as to leak through the TIR structure and blend the light with light leaking through surrounding portions of the light guide. The LEDs may be distributed only near the edges of the light guide or over the entire back surface of the light guide. A diffuser sheet may be laminated over the light guide to further mix the light.

FIELD OF THE INVENTION

This invention relates to a lighting structure comprising a light guidemixing and emitting the light from light emitting diodes (LEDs) and, inparticular, to forming structures in the light guide to optically couplethe LED light and spread the light. The structure may be used forbacklighting, general lighting or for other applications.

BACKGROUND

It is known to optically couple LED light into the side edges of atransparent light guide. A surface of the light guide has opticalfeatures, such as prisms or roughening, to allow the light to uniformlyleak out. Since the LEDs have a Lambertian emission, it is difficult tocouple into the edge of the light guide. Further, the light guide mustbe relatively thick to enable the edges to receive the light from theLEDs.

It is also known to form holes through the light guide around itsperimeter and position an LED at the bottom of each hole. An opaquereflector layer is then positioned over the top opening of each hole toreflect any top emission from the LED back into the light guide. Thereflector layer is typically a metallized plastic film. US patentapplication publication US 20080049445 describes a light guide with anLED positioned at the bottom of a hole with an opaque cusp-shapedreflector overlying each hole to reflect the light sideways into thelight guide. The light is further mixed in the light guide. Suchreflectors create dark spots over the LEDs and are difficult toprecisely align with the LEDs. An opaque bezel must be positioned overthe LEDs so the dark spots are not visible. Further, the reflector mayonly have a reflectivity of about 85-90%, so there is significantattenuation of light rays reflected off the reflective layer. Otherdisadvantages exist.

Additionally, since the LEDs are only positioned along the perimeter ofthe light guide, it is difficult to uniformly leak light out across thetop surface area of the light guide not covered by the bezel.

What is needed is a lighting structure that uses a light guide and LEDs,where the structure does not suffer from the drawbacks of the prior art.

SUMMARY

In one embodiment, a thin plastic light guide is formed to have cavitieson one surface and TIR (total internal reflection) structures directly(can be off-centered also) above the cavities. A through-hole is notformed. LEDs mounted on a printed circuit board (including a flexiblecircuit) are positioned in the cavities. The LEDs may be grouped, forinstance in one or more arrays. The LEDs may have a Lambertian emissionand no lens, so are very thin. At least the top ceiling of each cavityis shaped to direct the LED light to most efficiently make use of theTIR structures. For example, the TIR structures may have a cusp shape,and the top ceiling of the cavities may reflect the LED light tooptimally impinge on the TIR structure to maximize the TIR and themixing of light. Further, the TIR structures and cavities may be shapedto cause a controlled amount of light to leak out through the TIRstructures so there are no dark spots and no need for a bezel. Dependingon the configuration and function of the light guide the leakage throughthe TIR may be zero or small compared to the amount of light reflectedby the TIR structure.

The TIR structures and cavities may be symmetrical or asymmetrical. Anasymmetrical shape takes into account that the LEDs along a perimetershould have more of their light directed toward the middle of the lightguide. This technique can also be used to shape the beam or makingasymmetric light beam. The cavities may for instance be roughened,coated, filled with material and/or partly reflective to supportdirecting the light to the TIR structure.

The bottom or top surface of the light guide has additional opticalfeatures, such as tiny prisms, dots, or roughening, to cause the uniformleaking of light through the top surface. The optical features may varyalong the surface to leak out different percentages of the light toimprove uniformity across the light guide.

As mentioned above, the TIR structures and cavities may be formed toleak out a controlled amount of light so there are no dark spot over theLEDs. The light leaking through the TIR structure may be the samebrightness as the light leaking out other areas of the light guidesurface. This allows the LEDs to be distributed over the entire bottomsurface of the light guide, which improves uniformity and enables thelight guide to be any size and shape.

By molding the TIR structures in the light guide along with thecavities, the TIR structures and cavities can be precisely aligned. Thelight guide can possibly be very thin, even such as less than 3 mm, andflexible.

A diffuser sheet or other type of sheet may be laminated over the lightguide to further mix the light for increased uniformity or performanother function. A reflective film may be laminated with or withoutairgap on the bottom surface of the light guide and its edges to ensureall light exits the top surface of the light guide.

Other embodiments are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top down view of a light guide, in accordance with oneembodiment of the invention, showing areas where LED cavities and TIRstructures are located around the perimeter. The relative size of eacharea is greatly enlarged for clarity.

FIG. 2 is a bisected view of a single cavity and TIR structure (a cuspshape) in the light guide of FIG. 1, showing a simulation of light raysbeing shaped by the walls of the cavity and reflected of the TIRstructure, where only a miniscule amount of light is leaked through theTIR structure.

FIG. 3 is a bisected view of an alternative single cavity and TIRstructure (a cone shape), showing a simulation of light rays beingshaped by the walls of the cavity and reflected off the TIR structure,where a controlled percentage of light is leaked out through the TIRstructure so there is no dark or bright spot above the LED.

FIG. 4 is a bisected view of another single cavity and TIR structure (acusp shape), where a diffusion sheet or other type of sheet is laminatedover the light guide, and a reflector sheet is positioned below thelight guide.

FIG. 5 illustrates how the cavities and TIR structures can bedistributed over the entire area of the light guide, where the TIRstructures uniformly leak out light to blend with light leaked out fromsurrounding areas of the light guide, enabling the creation of a lightguide of any size and shape.

FIG. 6 illustrates optical features formed in a top or bottom surface ofthe light guide for leaking out light.

Elements that are the same or similar are labeled with the same numeral.

DETAILED DESCRIPTION

FIG. 1 illustrates a light guide 10 in accordance with one embodiment ofthe invention. The light guide 10 may be PMMA, PET, or other plastichaving a thickness on the order of for instance 2-5 mm. The light guide10 may be flexible.

The light guide 10 is molded, such as by pressing between two opposingheated rollers or plates having a negative of the molded features, toform top TIR structures 12 and bottom cavities 40. An LED 18 ispositioned in each cavity 40. The TIR structures 12 and cavities 40 areprecisely aligned due to the alignment of the molding rollers/plates.The TIR structures 12 and cavities are formed along one or more edges ofthe light guide 10 and are accordingly in this embodiment off centeredin the light guide. Also asymmetrical and off-centered configurations ofTIR structures and cavities with respect to the light guide arepossible. For a small light guide 10, only one TIR structure-cavity maybe required along only one edge. For a much larger light guide 10, theTIR structures 12 and cavities may be located along all four edges ordistributed over the entire surface. The pitch of the TIR structures 12and cavities depends on the number of LEDs needed for a desiredbrightness and the size of the light guide. Typical for this offcentered configuration of FIG. 1, the leakage through the TIR may bezero or small compared to the amount of light reflected by the TIRstructure.

Instead of molding the light guide and/or the cavities and/or the TIRstructures, one or more of techniques of injection molding, hotpressing, machining, 3D printing, rolling, laser treatment and othermanufacturing options may be used to create the light guide.

The LEDs may be grouped, for instance in one or more arrays, and mayhave same or different colors, for instance for color tuning, more inparticular for generating white light. In particular also neighboringLEDs may have different colors. (Multiple LEDs of same or differentcolors can also be used under one cavity as another type of solution).

In one embodiment, an opaque bezel 14, shown transparent with an openingdefined by the dashed-line rectangle, overlies the TIR structures 12 ifthere are dark spots or bright spots to be hidden. The bezel 14 may bepart of a frame that supports the light guide 10 or may be part of ahousing.

FIG. 2 is a bisected view of only a single TIR structure 12 and itsunderlying cavity 16. An LED 18 is mounted on a thin printed circuitboard 20 (PCB), which may be a flexible circuit, and the LED 18 ispositioned in the middle of the cavity 16. All the LEDs used may be onthe same PCB 20 and be aligned with all the cavities 16 for a simplefabrication process. The PCB 20 is fixed in position relative to thebottom surface of the light guide 10, such as via a frame.

In FIG. 2, the TIR structure 12 has a symmetrical circular cusp shape sothat light is reflected by TIR into the light guide 10. TIR structuresmay also have an asymmetrical and freeform shape. FIG. 2 shows simulatedlight rays 22 directed by the shaped walls of the cavity 16 and the TIRstructure 12 and also reflected by TIR off the smooth top and bottomsurfaces of the light guide 10. In an actual embodiment, the top orbottom surface of the light guide 10 has optical features for causingthe leaking out of a percentage of light across the top surface of thelight guide 10. Surface optical properties of top or bottom cavitysurfaces can also be tuned to control light leakage. If a diffuser sheetis used over the light guide, the emitted light will be uniform.

The LED 18 may be any type of LED, such as a GaN-based LED that emitsblue light, with one or more phosphor layers that are energized by theblue light to add red and green components to create white light. Someof the blue light leaks through the phosphor. The phosphor may be a YAGphosphor (emits green-yellow light) along with a red phosphor to achievethe desired color temperature. The LED emission is generally Lambertian.

In the example of FIG. 2, the light guide 10 is 3 mm thick, and thebezel 14 is 10 mm wide.

Light reflected by TIR is essentially not attenuated, while lightreflected by an opaque reflective film (such as a metalized layer) maybe attenuated by about 10-15%. Therefore, using the TIR structure 12 isan improvement over using an opaque reflective layer.

The cavity 16 shape (e.g., a convex, freeform, spline or polynomialshape) is tuned to the particular TIR structure 12 used. In FIG. 2, thecavity 16 shape refracts the LED 18 light to optimally shape the LEDemission for impinging on the cusp-shaped TIR structure 12 so that moreLED light is uniformly intercepted by the TIR structure at above thecritical angle for TIR. Therefore, the area over the TIR structure 12may be a dark spot. The LED side light is also redirected for improvedmixing in the light guide 10.

In the example, there is only a 0.1 mm gap between the top of the cavity16 and the point of the cusp. In other embodiments there may even be nogap and the top of the cavity 16 and the point of the cusp mayintersect.

Also shown in FIG. 2 is a thin reflective film 24, such as availablefrom Toray Industries, Inc., that is laminated over the PCB 20 andvertical edges 24 of the light guide 10 to reflect light back into thelight guide 10. A small air gap 26 is ideally created between thereflective film 24 and the light guide 10 to maximize TIR off the lightguide 10 surfaces, which is more efficient than reflection off thereflective film 24.

In one embodiment, the TIR structures 12 and cavities 16 are circularand symmetrical. In another embodiment, the TIR structures 12 andcavities 16 are asymmetrical so as to reflect more of the LED lighttoward the middle of the light guide 10 and less light toward the edges.This technique can also help to shape the beam, for example asymmetricbeam patterns.

In another embodiment, the TIR structures 12 and cavities 16 are shapedto leak out a controlled amount of light to blend in with thesurrounding areas of the light guide 10. In such a case, no bezel 14 isneeded to block dark or bright spots. However, in the example of FIG. 2,only a miniscule amount of light leaks through the point of the cusp.

FIG. 3 illustrates a different shape of the TIR structure 30 and cavity32. The TIR structure 30 is essentially cone shaped, allowing moreleakage due to more of the incident light being at less than thecritical angle. The shape of the cavity 32 is also tuned to the TIRstructure 30 to cause more of the LED light to impinge upon the TIRstructure 30 at less than the critical angle. The center portion of thecavity ceiling is generally flat so more of the LED light is directedtoward the center of the TIR structure 30 to escape through the TIRstructure 30. A 1 mm gap is shown between the top of the cavity 32 andthe TIR structure 30. The amount of light leaking through the TIRstructure 30 can be made uniform across the TIR structure 30 to matchthe brightness of light leaking through surrounding areas of the lightguide 36. In this way, no bezel is needed to hide any dark spots. Also,the LEDs may be arranged uniformly over the bottom surface of the lightguide 36 rather than only along the edges. This improves uniformity andenables the fabrication of a light guide of any size. Simulated lightrays 38 are shown. The remainder of FIG. 3 may be similar to FIG. 2.

FIG. 4 illustrates another cusp-shaped TIR structure 12 and analternative shape of the cavity 40. The cavity 40 has a top ceilingshaped such that it refracts more of the LED light at less than thecritical angle for the TIR structure 12 so that more LED light leaks outthe TIR structure 12.

FIG. 4 also illustrates the PCB 20, which may be a flexible circuit, aninsulated metal substrate, FR4, or other material. The PCB 20 has metaltraces that connect the anode and cathode terminals of the LEDs to apower source. A reflective layer 42 is provided over the PCB 20. Thereflective layer 42 may be a reflective solder mask. Ideally, there isan air gap between the reflective layer 42 and the bottom of the lightguide 43 so more light is reflected by TIR for improved efficiency(there can be optical contact (no airgap) also for some applications).The reflector layer 42 may be white to diffuse the light, or specular,or diffusing specular.

Over the top surface of the light guide 43 is laminated a diffusingsheet 44 or other type of sheet, such as an further optical structure,for instance using a transparent adhesion layer 46. The adhesion layer46, covering all or part of the top surface of the light guide, may forinstance be glue, tape or an index matching liquid. The diffusing sheet44 helps mix the light exiting the light guide 43 for increaseduniformity. The diffusing sheet 44 may contain light scatteringparticles and/or have optical features or a roughened surface. Suitablediffusing sheets are available from Toray Industries, Inc. Further, bynot having an air gap above the light guide surface, there is less TIRoff the surface (due to the higher index of refraction of the adhesionlayer 46 and diffusing sheet 44, and more light is extracted with fewerinternal reflections, improving efficiency. Ideally, the adhesion layer46 and diffusing sheet 44 have the same index as the light guide 43.

FIG. 5 illustrates how a light guide 50 (only a small portion is shown)may have the TIR structures 52 distributed across the light emittingsurface of the light guide 50. The TIR structures 52 and underlyingcavities are formed to leak out sufficient light to blend the leakedlight with the light leaking out the surrounding areas of the lightguide 50. Thus, a very large light guide 50 may be formed. Further, thelight guide 50 may be made thinner since there is less concern aboutlight attenuation by the light guide 50. The separation between LEDs 18may for instance be on the order of 10 mm-100 mm, depending on the typeof LED used, the desired brightness, and the size of the light guide.The relative sizes of the LEDs 18 and TIR structures 52 are greatlyexaggerated for clarity.

FIG. 6 illustrates a portion of the light guide 10 showing opticalfeatures 56 formed into the bottom of the light guide 10 to direct thelight (using TIR) toward the top surface of the light guide 10 at lessthan the critical angle (relative to the top surface) for escaping thetop surface. Multiple internal reflections are desirable to mix thelight from the LEDs. A reflective film 42 is used to reflect light backinto the light guide 10. A light ray 58 is shown.

The optical features 56 may vary along the surface to most uniformlycause light to leak out across the light guide.

In another embodiment, roughening the light exit surface of the lightguide 10 results in the controlled leakage of light.

Accordingly, there may be five optical surfaces in the structure thatdirect light: the cavities, the TIR structures, the TIR of the smoothlight guide surfaces, the light extraction features, and the diffusingsheet.

In one embodiment, the LED light coupling efficiency into the lightguide is about 96%. The index of refraction of the PMMA light guide usedin the simulations is about 1.4936.

In another embodiment, the cavities and TIR structures may be other thancircular, such as square shaped. Also the light guide may have differentshapes such as rectangular, square, round or any irregular/asymmetricshape.

In one embodiment, the resulting light guide provides illumination as adown light, for instance for general illumination or for horticultureapplication. In another embodiment, the light guide serves as abacklight for an LCD display, such as in a monitor, smart phone display,or television. Since the LEDs used in the embodiments do not require alens, the LEDs are very thin, enabling the formation of a very thin(less than 3 mm) backlight for use in a smart phone. Other embodimentsof the invention may be used for automotive applications or in photocamera flashes, for instance in smart phones. Depending on the specificapplications one or more LEDs may generate light in the UV or infraredspectrum. Although the LEDs in the shown embodiments are top-emitters,within the scope of the invention also other types of LEDs may be used,such as side-emitters and n-sided emitters.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is being claimed is:
 1. An illumination structure, comprising: alight guide configured to receive light emitted in an angulardistribution that is substantially centered around a central axis andguide the received light between a first surface of the light guide andan opposing second surface of the light guide, the first surfaceincluding a cavity formed in the first surface, the cavity having aconvex central portion that is substantially centered around the centralaxis, the convex central portion configured to refract at least some ofthe emitted light into the light guide as refracted light, the secondsurface including a total internal reflection (TIR) structure that isshaped to reflect at least some of the refracted light via TIR andpropagate the reflected light in the light guide generally away from thecentral axis.
 2. The illumination structure of claim 1, wherein: thefirst surface extends substantially in a first plane; and the secondsurface extends substantially in a second plane.
 3. The illuminationstructure of claim 2, wherein the first plane and the second plane aregenerally orthogonal to the central axis.
 4. The illumination structureof claim 3, wherein the cavity and the TIR structure extend into thelight guide such that the light guide is fully located between the firstplane and the second plane.
 5. The illumination structure of claim 2,further comprising a light-emitting diode configured to emit the lightin the angular distribution that is substantially centered around thecentral axis, the light-emitting diode being at least partially locatedin the cavity between the first plane and the second plane.
 6. Theillumination structure of claim 1, wherein the convex central portion isgenerally orthogonal to the central axis at a center of the convexcentral portion.
 7. The illumination structure of claim 1, wherein theconvex central portion adjoins a side wall of the cavity at a cornerthat encircles the convex central portion.
 8. The illumination structureof claim 1, wherein the TIR structure is shaped as a portion of a cone.9. The illumination structure of claim 1, wherein a cross-section of theTIR structure, taken in a plane that includes the central axis, definesa curve that has a central region that intersects the central axis andconvex portions on opposite sides of the central region.
 10. Theillumination structure of claim 1, further comprising a diffusing sheetoverlying the second surface of the light guide.
 11. The illuminationstructure of claim 1, wherein the light guide has light extractionfeatures formed on the first surface.
 12. The illumination structure ofclaim 1, wherein the light guide has light extraction features formed onthe second surface.
 13. A method of producing illumination, the methodcomprising: receiving, with a light guide, emitted light having anangular distribution that is substantially centered around a centralaxis; and guiding the received light between a first surface of thelight guide and an opposing second surface of the light guide, the firstsurface including a cavity formed in the first surface, the cavityhaving a convex central portion that is substantially centered aroundthe central axis, the second surface including a total internalreflection (TIR) structure; refracting, with the convex central portion,at least some of the emitted light into the light guide as refractedlight; and reflecting, via TIR from the TIR structure, at least some ofthe refracted light to propagate in the light guide generally away fromthe central axis.
 14. The method of claim 13, wherein: the first surfaceextends substantially in a first plane; the second surface extendssubstantially in a second plane; the first plane and the second planeare generally orthogonal to the central axis; the cavity and the TIRstructure extend into the light guide such that the light guide is fullylocated between the first plane and the second plane; the light-emittingdiode is at least partially located in the cavity between the firstplane and the second plane; the convex central portion is generallyorthogonal to the central axis at a center of the convex centralportion; and the convex central portion adjoins a side wall of thecavity at a corner that encircles the convex central portion.
 15. Themethod of claim 13, further comprising emitting the emitted light from alight-emitting diode.
 16. An illumination structure, comprising: a lightguide configured to receive light emitted in an angular distributionthat is substantially centered around a central axis and guide thereceived light between a first surface of the light guide and anopposing second surface of the light guide, the first surface includinga cavity formed in the first surface, the cavity having a convex centralportion that is substantially centered around the central axis and isgenerally orthogonal to the central axis at a center of the convexcentral portion, the convex central portion adjoining a side wall of thecavity at a corner that encircles the convex central portion, the convexcentral portion configured to refract at least some of the emitted lightinto the light guide as refracted light, the second surface including atotal internal reflection (TIR) structure that is shaped to reflect atleast some of the refracted light via TIR to propagate in the lightguide generally away from the central axis, a cross-section of the TIRstructure, taken in a plane that includes the central axis, defining acurve that has a central region that intersects the central axis andconvex portions on opposite sides of the central region; and a diffusingsheet overlying the second surface.
 17. The illumination structure ofclaim 16, wherein: the first surface extends substantially in a firstplane; the second surface extends substantially in a second plane; thefirst plane and the second plane are generally orthogonal to the centralaxis;
 18. The illumination structure of claim 17, wherein the cavity andthe TIR structure extend into the light guide such that the light guideis fully located between the first plane and the second plane.
 19. Theillumination structure of claim 17, further comprising a light-emittingdiode configured to emit the light in the angular distribution that issubstantially centered around the central axis, the light-emitting diodebeing at least partially located in the cavity between the first planeand the second plane.
 20. The illumination structure of claim 16,wherein the TIR structure is shaped as a portion of a cone.